Ohio's age of consent for sexual acts is 16 if the other person is 18 years or older: http://codes.ohio.gov/orc/2907.04
Unless the 18 year old is mentally retarded or your sister has a fiduciary relationship with the 18 year old (i.e., she is his teacher, lawyer, doctor, etc), being with him shouldn't constitute a criminal offense.
Your sister should speak privately with her family law attorney about her situation. No one should post anything about her situation online.
By the way, even though her being with an 18 year old may not constitute a criminal offense, which parent gets custody is determined by the "best interests of the child." Many things--even things that aren't criminal acts--can be taken into consideration.
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It is not criminal behavior. However, someone can try to lose a lot of things in the divorce case to show that the other person is not a fit parent or that living with the parent is not in the best interest of the child. I suggest she hire a family law attorney if she hasn't already. Trying to fix the screwups that a client gets themselves in representing themselves is always harder and more expensive than getting it right the first time.
Dayton, Ohio Criminal Defense Attorney
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